The experiments conducted in the first year have established that the mouse blastocyst does exert some local, uterotrophic effect on the uterus, as shown by an increase in weight and in the uptake of amino acids. The studies proposed for next year will be to determine: (1) whether the blastocyst can affect the local uptake of estrogen in the uterus; (2) whether the uterotrophic effect of blastocysts is increased with the elapsed time and the number of blastocysts present in the uterus; and (3) the nature of blastocyst action on the uterus, e.g., chemical or physical. Our data also suggested that rabbit blastocysts can synthesize estradiol-17beta from testosterone in vitro. Further studies will be: (1) to substantiate and quantify the estrogen synthesis capacity of rabbit blastocysts, with estradiol-17beta to be crystallized to constant specific activity, (2) to determine whether synthesis of estrogen by rabbit blastocysts can be suppressed by aromatase inhibitors, and (3) to inhibit implantation in rabbit with another aromatase inhibitor, 4-acetoxy-androstenedione, (namely, 4-acetoxy-4-androstene-3,17-dione).